Among conventional metallic gaskets aimed at improving the cooling effects of the parts between which a metallic gasket is sandwiched, there is one which is described in Japanese Patent Laid-Open No. 60-3465.
As shown in FIGS. 21 and 22, this metallic gasket comprises a base plate 50 made of a thin, high-rigidity, elastic metal plate having a first metal bead 52 formed along a seal line SL1 encircling the inner peripheral edge of each combustion chamber opening 51. In the base plate 50, on the outer side of the first metal bead 52, a second metal bead 55 is formed along a seal line SL2 encircling cooling water holes 53 but separating and not encircling bolt holes 54. Reference numeral 56 denotes an oil hole, and a third seal line SL3 is formed encircling the inner peripheral edge of the oil hole 56.
When the metallic gasket is mounted at the joint between the joint surface of a cylinder block 60 and the joint surface of a cylinder head 61 and fastened with bolts, the metal beads 52, 55 are elastically deformed according to a fastening force, a combustion gas of the combustion chamber bore 51 is line-sealed by the first metal bead 52 and a gap space S at a minute height, to which only the cooling water holes 53 are open, is formed between the two metal beads 52, 55. Cooling water is pressure-fed through the cooling water holes 53 into the gap space S when the engine is running, and as a result of cooling water circulation, the joint surfaces and the metallic gasket itself are cooled. Also by the pumping action of vibration amplitude caused by explosion of the combustion gas, the flow of cooling water to the gap space S is promoted.
The minute gap of the gap space S is formed by a balance of the axial tension of the clamping bolts of the engine with the urging force of the metal beads 52, 55 that resist the fastening. Since a line seal by metal is applied, it is necessary to set a high gasket factor to secure a desired sealing property, so that the axial tension of the bolts must be set on a higher side, and the gap tends to become small.
Further, the cubical expansion of the engine occurs by the heat of engine operation, increasing the fastening pressure, and as the temperature rises, the above-mentioned gap decreases. Progresses have been made in reductions in size and weight, technical sophistication, and energy saving of the engine, and aluminum has come to be used in greater quantities as material for the engines. As a result of increased expansion coefficient by a difference of materials, there is a tendency that it is becoming difficult to secure the above-mentioned gap.
Further, in the bead structure of the above-mentioned gasket, as described above, the metal beads 52, 55 formed in the base plate 50 need to generate a high surface pressure. Therefore, it is necessary to adopt a material of high hardness, and in hard materials, internal stress concentrates in the portions of bending radius of the metal beads 52, 55, and when subjected to repetitive stress by vibration amplitude, they are liable to fatigue failure, and gasket lifetime is shortened.
Supposing an engine is made by aluminum, casting of aluminum is difficult and blowholes occur in casting. Therefore, when processing the joint surfaces, adjacent blowholes on the joint surfaces are sometimes connected. At this time, the conventional beads 52, 55 are for line sealing and come into metal-to-metal contact with the joint surfaces, and therefore the blowholes may stretch, running across the line-sealed bead line, or the seal line may be displaced during operation due to an expansion difference between materials of the gasket and the engine, resulting in the beads 52, 55 being located on the blowholes, increasing chances of water leakage. An aluminum-made engine is liable to dents in transit. Those dents may give rise to the above-mentioned phenomena.
Further, since the base plate 50 needs to be formed by a material of high hardness, it is difficult to improve the processing accuracy of the metal beads 52, 55 formed by bending the base plate 50.
Among other conventional metallic gaskets, there is one which is disclosed in Japanese Patent Laid-Open No. 2001-173791.
As shown in FIG. 23, this metallic gasket comprises two base plates 50. In other words, a thickness-increased portion 52 is formed at the inner peripheral end of the combustion chamber opening 51 side of the thicker base plate (the upper base plate) out of the two base plates 50, convex beads 53 are formed on the base plates 50, each on the outer side of the thickness-increased portion 52 and at a height higher than the thickness of the thickness-increased portion 52, and the two base plates 50 are laminated in such a way that the convex sides of the base-plate beads 53 face each other. Further, an elastic sealing material 54 is filled in the outside-facing concave portions of the metal bead 53.
The metallic gasket is disposed between the opposing joint surfaces of the cylinder head and the cylinder block, and when they are fastened together, the base-plate beads 53 are compressed and deformed until the thickness-increased portion abuts on the opposite base plate at the peripheral edge of the combustion chamber opening, concurrently with which, the elastic sealing material parts 54 filled in the concave portion are compressed and deformed, and consequently a combustion gas, oil, and cooling water are sealed by a sealing pressure from a combined spring including the spring force of the base-plate beads 53 and the spring force of the elastic sealing material parts 54. Needless to say, some of the conventional metal beads have no elastic sealing material filled in their concave portion and some conventional metal beads are formed by a single piece of base plate.
In this conventional metallic gasket, when the base-plate beads 53 and the elastic sealing material parts 54 are deformed when the bolts are tightened, they cooperate to generate a resilience to thereby generate a required sealing pressure along the seal lines.
However, when the base plate 50 is formed of metal plate of low hardness with a view to preventing fatigue failure of the base-plate beads 53 and reducing production cost, in the above-mentioned metallic gasket, when the bolts are fastened and the elastic sealing material 54 in the concave portion is deformed under compression, an external force acts to deform the base plate 50 and the base-plate beads 53 to warp in the through-thickness direction. When the base plate 50 is made of metal of low hardness as mentioned above, the bead shape-preserving power is low and the base plate deformation resistance is weak, so that the sealing property is reduced accordingly.
By repeated load by repetition of operation and stoppage of the engine, after a long period of use, problems arise, such as a decrease in axial tension of the clamping bolts, changes with time of the base-plate bead 53 on the base plate 50, or deterioration in the elastic sealing material 54 of the concave portion of the bead; therefore, the seal surface pressure is likely to drop. Such problems tend to come up particularly at overhanging parts on the outer side of the clamping bolts.
When the elastic sealing material 54 is formed by baking in the concave portions of the base plates, even if the elastic sealing material 54 at high temperature is filled in the concave portions, it changes in volume by an amount of thermal expansion during subsequent open cooling, the center portion of the elastic sealing material 54 where the thickness is at its highest contracts by an amount of thermal shrinkage. This is disadvantageous when the surface pressure decreases as described above. Such a phenomenon as this seems to be likely to occur particularly when the gasket is mounted in the engine which has been assembled with a weak fastening axial tension.